目前，各个语言基础库中都会实现的Map<Key, Value>字典类，经常有两种实现方式，一种是基于哈希思想的，一种是基于平衡树或者跳表这样的有序集合；其本质可以认为就是实现了分门别类的功能。因此，对于group by聚集这样的要求，也有两种主流的实现方式；一种是基于哈希表，我们称为哈希聚集；另一种则要求先对元组...
totalCount / pageSize : (totalCount / pageSize) + 1; this.next = pageNow + 1; this.last = pageNow - 1; if (this.totalPage == 1) { this.isFirst = true; this.isLast = true; this.hasLast = false; this.hasNext = false; } else { if (this.pageNow <= 1) { this.pageNow =...
通过first_value()窗口函数很容易查询分组数据的最大值或最小值,例如score表按课程分组同时取每门课程的最高分,如下所示: mydb=> SELECT first_value(score) OVER( PARTITION BY subject ORDER BY score desc),* FROM score; first_value | id | subject | stu_name | score ---+---+---+---+--...
SELECT select_listFROMtable_expressionORDERBY sort_expression1[ASC | DESC][NULLS { FIRST | LAST }][, sort_expression2 [ASC | DESC][NULLS { FIRST | LAST }]...] NULLS FIRST 和 NULLS LAST 选项可用于确定在排序顺序中空值是出现在非空值之前还是之后。默认情况下,空值排序为好像大于任何非空值;...
merges two record sets by looping through every record in the first set and trying to find a match in the second set. All matching records are returned. 遍历模式 索引模式 5、聚合 - Aggregate group by 操作 groups records together based on a GROUP BY or aggregate function (like sum()). ...
select left(stringu1, 1) as FirstLetter, avg(ten) from tenk1 group by left(stringu1, 1); firstletter | avg ---+--- N | 5.0000000000000000 O | 4.0000000000000000 V | 5.0000000000000000 L | 5.0000000000000000 Z | 4.9895833333333333 M | 4.0000000000000000...
目前言 MySQL函数聚合函数数学函数字符串函数日期函数控制流函数窗口函数序号函数开窗聚合函数- SUM,AVG,MIN,MAX 前后函数 lag lead 首尾函数first_value...SELECT语句及其条件表达式都可以使用这些函数,函数可以帮助用户更加方便的处理表中的数据,使MySQL数据库的功能更加强大。本篇文章主要为大家介绍几类常用函数的用法...
FirstTime String 最早一条慢SQL时间示例值:2024-10-01T10:00:00Z LastTime String 最晚一条慢SQL时间示例值:2024-10-01T10:05:00Z SharedReadBlks Integer 读共享内存块数示例值:300 SharedWriteBlks Integer 写共享内存块数示例值:50 ReadCostTime Integer 读io总耗时示例值:100 WriteCostTime Integer 写...
聚合函数是从一组输入中计算出一个结果的函数。 测试表 test=# \d tbl_testTable"public.tbl_test"Column|Type|Modifiers---+---+---id|integer|name|charactervarying(32)|sex|charactervarying(1)|test=#select*fromtbl_test; id|name|sex---+---+---1|张三|m2|李四|m3|...
COALESCE –return the first non-null argument. You can use it to substitute NULL by a default value. NULLIF –return NULL if the first argument equals the second one. CAST –convert from one data type into another e.g., from a string into an integer, from a string into a date. Secti...